Methods of treating organic wastes in anaerobic biological reactors, containing methanogenic and other anaerobic bacteria in suspended floc form are known. These methods involve the use of closed reaction vessels or digesters/reactors (hereinafter referred to as reactors) which are usually operated at a temperature of about 35.degree. C. The suspended microbial biomass (reactor liquor) converts the biodegradable material in the influent wastes to biogas (a mixture of methane, carbon dioxide and minor amounts of other gases) and produces also a microbial cell residue (digested sludge). The biogas bubbles grow on the surfaces of liquor suspended solids, from which they are separated by the shearing action of a mixing system. The mixing system is generally related to the shape of the reactor. Reactors are generally of cylindrical construction with a domed roof and conical floor. Known methods of mixing include mechanical stirring by turbine or propellor, mixing by pumped recirculation of the liquor or gas mixing. The latter method involves recycling biogas from a gas pocket formed between the surface of the liquor and the reactor roof through one or more submerged outlets using a gas blower.
All of the known methods of mixing require mechanical equipment within the closed reactor. It is not feasible to inspect or carry out routine maintenance on such internally located equipment. For example, turbines and propellors may become fouled by fibrous materials; submerged gas manifold orifices may become clogged. The known systems also use significant amounts of energy in their operation.